Disk arrays comprising a multiplicity of small, inexpensive disk drives connected in parallel have emerged as a low cost alternative to the use of single large disks for non-volatile storage of information within a computer system. The disk array appears as a single large fast disk to the host system but offers improvements in performance, reliability, power consumption and scalability over a single large magnetic disk.
Several disk array design alternatives were presented in an article titled "A Case for Redundant Arrays of Inexpensive Disks (RAID)" by David A. Patterson, Garth Gibson and Randy H. Katz; University of California Report No. UCB/CSD 87/391, December 1987. The article, incorporated herein by reference, describes five disk array arrangements, referred to as RAID levels. The first level RAID comprises N disks for storing data and N additional "mirror" disks for storing copies of the information written to the data disks. RAID level 1 write functions require that data be written to two disks, the second "mirror" disk receiving the same information provided to the first disk. When data is read, it can be read from either disk.
RAID level 3 systems comprise one or more groups of N+1 disks. Within each group, N disks are used to store data, and the additional disk is utilized to store parity information. During RAID level 3 write functions, each block of data is divided into N portions for storage among the N data disks. The corresponding parity information is written to a dedicated parity disk. When data is read, all N data disks must be accessed. The parity disk is used to reconstruct information in the event of a disk failure.
RAID level 4 systems are also comprised of one or more groups of N+1 disks wherein N disks are used to store data, and the additional disk is utilized to store parity information. RAID level 4 systems differ from RAID level 3 systems in that data to be saved is divided into larger portions, consisting of one or many blocks of data, for storage among the disks. Writes still require access to two disks, i.e., one of the N data disks and the parity disk. In a similar fashion, read operations typically need only access a single one of the N data disks, unless the data to be read exceeds the block length stored on each disk. As with RAID level 3 systems, the parity disk is used to reconstruct information in the event of a disk failure.
RAID level 5 is similar to RAID level 4 except that parity information, in addition to the data, is distributed across the N+1 disks in each group. Although each group contains N+1 disks, each disk includes some blocks for storing data and some blocks for storing parity information. Where parity information is stored is controlled by an algorithm implemented by the user. As in RAID level 4 systems, RAID level 5 writes require access to at least two disks; however, no longer does every write to a group require access to the same dedicated parity disk, as in RAID level 4 systems. This feature provides the opportunity to perform concurrent write operations.
An additional disk array arrangement, referred to as parity striping, is presented in an article titled "Parity Striping of Disc Arrays: Low-Cost Reliable Storage with Acceptable Throughput" by Jim Gray, Bob Horst and Mark Walker; Tandem Computers, Inc., Tandem Technical Report No. 90.2, January 1990. In the parity striping system, only parity information is distributed across the disks, and the parity is mapped as large contiguous extents. Data is not divided among the disks but is stored in the conventional manner.
To utilize any of the disk array configurations described above within a computer system, a data path structure interconnecting the host system data busses with the plurality of disk drives is required. Most systems include a fixed data path structure interconnecting the plurality of disk drives with the host system. However, such a fixed structure frustrates rearrangement of the disk array system to accommodate different quantities of disk drives or different RAID configurations. Additionally, in most disk array systems the host operates as the array controller and performs parity generation and checking, increasing host processing overhead.